EXCESS PROPERTIES AND PHASE-EQUILIBRIA CALCULATED FROM A GENERALIZED VAN-DER-WAALS-EQUATION OF STATE FOR THE EXAMPLE OF THE MIXTURE METHANEPLUS ETHANE

A generalized van der Waals equation of state, formulated recently for pure non-polar substances, is now adapted for the treatment of mixtur es. The main contribution to the van der Waals attractive term, the cr itical temperature T-c, is now considered as a mixture parameter for t he equivalent substance (the one-fluid mixture model). It is determine d from the orthobaric liquid density of the mixture, i.e. from the exc ess volume. All other parameters are supposed to be additive in mole f raction. With these mixing rules, the equation gives good agreement fo r the excess Gibbs energy of methane + ethane over a larger temperatur e range. The phase equilibria are calculated according to the algorith m of Deiters (1985) and cover the whole range of temperatures and comp osition up to the critical curve. Apart from the immediate neighborhoo d of the critical points of the mixtures, the agreement is very satisf actory. The densities in the homogeneous region and the interaction se cond virial coefficients are also well reproduced.